The invention concerns an automatic clutch in the drive train of a motor vehicle, which has a transmission which is arranged between the engine and the drive wheels and can be shifted arbitrarily between gears or drive positions with different transmission ratios. A motorized adjusting unit actuates the clutch and is controlled by a system of sensors which senses the predetermined parameters. The system of sensors senses parameters correlated on the one hand with the torque of the engine and on the other hand with the transmissible moment of the clutch, and control the clutch with parameter-dependent excess contact pressure during driving.
In the case of motor vehicles with customary internal combustion engines, a transmission is required in the drive train to allow the transmission ratio between the speed of the vehicle engine and the speed of the drive wheels to be changed according to the respective travelling speed and loading of the vehicle. In the case of customary manually shifted transmissions, during a gear change or change of drive position, the power flow between the engine and the drive wheels has to be interrupted by releasing a clutch.
Automatic clutches are already standardly used for this purpose, i.e. the clutch is automatically released when changing gear or drive position and is subsequently re-engaged.
In addition, the clutch is also automatically controlled when starting the vehicle. If a drive position suitable for starting is selected, the clutch can, for example, be automatically engaged lightly in such a way that a very small torque adequate for making the vehicle begin to creep forward is transmitted. As soon as the engine power and the engine speed are increased by actuating a gas pedal or the like, the clutch is automatically engaged increasingly in order to increase the transmissible torque.
These known automatic clutches operate in the manner of friction clutches, the frictional engagement of which is controlled by the assigned adjusting unit of the clutch. In this case, the adjusting unit operates against a spring arrangement acting on the clutch so as to make it engage, i.e. the adjusting unit must generate varying degrees of counteracting force against the spring element if the frictional engagement or the transmissible moment of the clutch is to be reduced. When there is maximum frictional engagement or maximum transmissible moment of the clutch, the adjusting unit may be without any force at all. When the frictional engagement or the transmissible moment of the clutch is reduced, the adjusting unit must, furthermore, travel a varying distance. In the state of maximum frictional engagement or maximum transmissible moment of the clutch, the adjusting unit assumes an end position.
German Patent document DE-A 1 505 469 discloses an electromagnetically actuated clutch of the above-mentioned type.
The object of the invention is thus to present a particularly advantageous apparatus for controlling the automatic clutch.
This object is achieved according to the invention by the transmissible clutch moment exceeding the respective engine torque by a smaller amount in gears or drive positions with a high transmission ratio between the engine speed and the speed of the drive wheels and/or at low driving speed than in gears or drive positions with a lower transmission ratio and/or at higher driving speed.
The invention is based on the general idea of controlling the moment which can be transmitted by the clutch in a way analogous to the torque respectively generated by the engine and thereby operating with variable excess contact pressure of the clutch in adaptation to the respective operating conditions of the vehicle, i.e. the moment which can be transmitted by the clutch exceeds the torque respectively generated by the engine to a variable degree. It is envisaged here to decrease the excess contact pressure significantly when the risk or probability of shocks in the drive train--e.g. load cycle shocks--is particularly great, for example at low driving speed and/or when driving in low gears or drive positions with high transmission between the engine speed and the speed of the drive wheels. This, at the same time, takes into account the fact that in such driving situations gear changes or changes in drive position are performed comparatively frequently. On account of the low excess contact pressure, the adjusting unit can release the clutch particularly quickly, since a state with low excess contact pressure is synonymous with the adjusting unit being at a distance equivalent to varying travel from its end position, which is assumed when there is the maximum transmissible moment of the clutch.
If the transmission is shifted into a drive position or into a gear in which there is a low transmission ratio between the engine speed and the speed of the drive wheels, or in driving situations at high speed, the invention provides on the other hand that the excess contact pressure can be increased up to the maximum moment which can be transmitted by the clutch. In such operating phases, appreciable shocks in the drive train are unlikely. There is therefore no requirement to reduce the clutch moment or the frictional engagement of the clutch to a level at which the clutch, can for a short time, operate with slip, i.e. can slip, in the case of such shocks in order to absorb them. By setting the maximum transmissible moment or frictional engagement of the clutch, the adjusting unit is relieved in a desirable way, in particular the adjusting unit can be significantly saved in this way during long journeys on high speed roads.
Otherwise, with regard to preferred features of the invention, reference is made to the claims and the following explanation of the drawing, on the basis of which particularly preferred embodiments of the invention are described.